It is known that, in this case, the autothrust system automatically regulates the rating of the engines, in a continuous manner, as a function of the deviation between the measured speed of the aircraft and the preset speed. Such an autothrust system therefore makes it possible to produce a stabilized (on the preset speed) speed of the aircraft.
However, by reason of the generation of a continuous (or permanent) variation of the rating of the engine or engines, which gives rise to a permanent variation of the sound frequencies emitted by the engines (the sound frequencies varying of course with the rating of the engines) this autothrust system therefore has the drawback of causing a permanent variation in the noise emitted by the engine or engines, thereby reducing acoustic comfort, in particular of the passengers of the aircraft. However, on a transport airplane in particular, the comfort of the passengers is essential during a phase of horizontal flight at stabilized speed such as a cruising phase, which may last several hours.